1. An elongated plant cell whose walls are extensively (usually completely) thickened with lignin (see sclerenchyma). Fibres are found in the vascular tissue, usually in the xylem, where they provide structural support. The term is often used loosely to mean any kind of xylem element. The fibres of many species, e.g. flax, are of commercial importance.

2. Any of various threadlike structures in the animal body, such as a muscle fibre, a nerve fibre, a collagen fibre, or an elastic fibre.

3. (dietary fibre; roughage) The part of food that cannot be digested and absorbed to produce energy. Dietary fibre falls into four groups: cellulose, hemicelluloses, lignins, and pectins. Highly refined foods, such as sucrose, contain no dietary fibre. Foods with a high fibre content include wholemeal cereals and flour, root vegetables, nuts, and fruit. In human nutrition, a substantial proportion of dietary fibre consists of so-called non-starch polysaccharides (cellulosic and non-cellulosic polysaccharides excluding resistant starch). Another distinction is between soluble and insoluble fibre. Soluble fibre (e.g. in oats, pulses, fruit, vegetables) is broken down by bacteria in the large intestine to yield short-chain fatty acids, some of which can be absorbed and metabolized by the liver. Insoluble fibre (e.g. in wholegrain cereals) is resistant to bacterial attack and, with the bacterial cells, forms a bulky water-retaining mass that promotes gut peristalsis and accelerates the passage of digesta and faeces. Dietary fibre is considered by some to be helpful in the prevention of many of the diseases of Western civilization, such as diverticulosis, constipation, appendicitis, obesity, and diabetes mellitus.